Method and apparatus for grid loss ride-through for wind turbine pitch control system

ABSTRACT

In a wind turbine/generator having a rotatable hub, at least one blade rotatably secured to the hub, a pitch control system for adjusting pitch of each blade, the pitch control system located within the rotatable hub, a stationary nacelle, and a slip ring assembly at a junction of an electrical circuit between the rotatable hub and the stationary nacelle, the slip ring assembly operatively arranged for transmission of electrical signals between equipment located within the rotating hub and equipment located within the stationary nacelle, an apparatus for grid loss ride-through for the pitch control system, comprising for example, operational amplifiers for sensing and monitoring power on the rotating side of the slip ring assembly, and, a contactor coil, for example, for supplying power to the pitch control system from a backup power source when the sensed power drops to a predetermined level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Application No.PCT/US2007/015854, filed Jul. 12, 2007 which application is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates generally to wind turbines, morespecifically to pitch control systems for wind turbines, and, even morespecifically, to a method and apparatus for grid-loss ride-through for awind turbine pitch control system.

BACKGROUND ART

As is well known, a wind turbine is a machine that converts the kineticenergy in wind into mechanical energy. If the mechanical energy is useddirectly by machinery, such as a pump or grinding stones, the machine isusually called a windmill. If the mechanical energy is converted toelectricity, the machine is called a wind generator. Wikipedia,http://en.wkipedia.org/wiki/Wind_turbine. Wind turbines can be furthercategorized by structure and orientation based on the axis about whichthe turbine rotates. Turbines that rotate about a horizontal axis arecalled horizontal-axis wind turbines (HAWT), whereas those that rotateabout a vertical axis are called vertical-axis wind turbines (VAWT).HAWTs are more common than VAWTs. Wikipedia, supra.

In principle, producing electric power with wind is a simple process.Most HAWT turbines have three large blades mounted to a rotating hub.The blades are aerodynamically designed to turn as easily as possiblewhen the wind blows on them (the number of blades may vary). The turningblades spin a shaft, which connects through a gearbox to a generatorthat produces electricity. The gearbox and generator are mounted in anacelle which, in turn, is mounted atop a tower. As the wind blows overthe turbine blades they create “lift”, much like an airplane wing, andbegin to turn. The spinning blades turn a low-speed shaft at arelatively low speed, usually 30-60 rpm. The gearbox connects thelow-speed shaft with a high-speed shaft that drives the generator. Thegearing also boosts the rotation speed of the high-speed shaft to theoperating speed of the generator. This operating speed may vary, but isusually in the range of 900-1800 rpm. This rapidly spinning shaft drivesthe generator to produce electric power. The generator's electricaloutput is connected to the larger electrical grid. Typically, largecapacity generators provide polyphase voltages at a controlled frequencysynchronized to the grid. The generator outputs are connected to thegrid via suitable transformers.

The blades themselves can also be turned, or pitched, about theirlongitudinal axes, out of the wind, to control the rotor speed and keepthe rotor from turning in winds that are too high or too low to produceelectricity. They can also be pitched to a “feather” position to preventrotation in the event of an emergency. (The wind turbine also typicallyincludes an emergency braking system to stop rotation in the event of anemergency.) The blades are rotated about their longitudinal axes by apitch control system. There are several different ways of doing this,including actuators and motors. The pitch control system, whichcomprises motors or actuators and associated power supplies and controlelectronics, is conventionally mounted in the rotating hub of theturbine. Power is supplied to the pitch control system from slip ringswhich transmit power from a stationary bus/supply mounted in thenacelle. The power supply for the pitch control system can come from anumber of sources. It can be provided by the main grid itself viaappropriate transformers, or it can be provided by the generator drivenby the turbine.

Historically, wind turbines have contributed a very low percentage ofthe world's energy demands. But depletion of natural resources such asoil and natural gas, associated higher prices for these resources, andpolitical ramifications associated with reliance on foreign oil, arechanging the energy generation landscape. The industry is respondingwith turbines of higher capacities (ratings of 1.5 MW or more), bettertechnology, and wind farms having large numbers of wind turbines. Asrecently reported by CNNMoney.com, “Wind energy industry sourcesreported that approximately 15,000 megawatts of new wind energygeneration capacity was installed worldwide in 2006, an increase of 25percent from 2005. The industry has maintained an average growth rate ofmore than 17% for the past five years, and industry estimates project asimilar growth rate and a total wind energy equipment market value ofmore than $180 billion for the next five years.”http://money.cnn.com/news/newsfeeds/articles/prnewswire/LAM00302072007-1.htm.These statistics and forecasts are confirmed by E.ON Netz, the Germantransmission system operator of the E.ON Group, who reported in 2005,“In 2004, Germany was once again the global world leader in theproduction of wind power. At the end of 2004, wind energy plants with aninstalled capacity of 16,400 MW supplied the German electricity grids .. . . According to grid studies by the Deutsche Energie-Agentur (dena),wind power capacity in Germany is expected to increase to 48,000 MW by2020, around a threefold increase since 2004 . . . . This means thatGermany remains the world's undisputed number one generator of windenergy. In 2004, Germany accounted for approximately one third of theworld's and half of Europe's wind power capacities . . . . In total,German wind farms generated 26 billion kWh of electricity, which isaround 4.7% of Germany's gross demand.” Wind Report 2005, E.ON Netz. Inthe past, when wind turbines played a negligible role in powergeneration, they could be largely ignored when considering gridstability. This is no longer the case.

In response to this growth in the wind turbine industry and its impacton the national grid, the Federal Energy Regulatory Commission (“FERC”)has proposed minimum requirements for wind plant response to certainlow-voltage conditions on the utility power grid. These requirementsrequire that wind turbines stay connected to the grid during prescribedtransient “grid-loss” conditions. Similar requirements are beingmandated by grid connection and regulatory authorities throughout theworld. Generally, they describe the voltage falling immediately at t=0to a substantially reduced level such as 10 or 15% of nominal line leveland then gradually returning to at least 80% of nominal line levelwithin three seconds of t=0. The levels are considered to be all threephases combined and not with regard to the individual phases. Theaggregated requirements of FERC, E.ON Netz (Germany), HECO (Hawaii), andthe Spanish grid authority, for example, can all be satisfied by onesimplified power loss profile described as follows: the pitch controlsystem should continue to operate normally when the AC mains voltagelevel falls below 80%, and as low as zero, and remains below 80% for atleast as long as three seconds, at which time the AC main level returnsto a minimum of 80% of nominal line level.

This continued operation of the pitch control system is referred to inthe industry as “ride-through” capability. It broadly describes theability of the pitch control system to function during a “grid loss”condition, i.e., a condition which cuts power to the pitch controlsystem for any number of reasons. Interestingly, not everyone in theindustry defines “grid loss” in the same way, or attempts to solve thesame problem, much less in the same way. For purposes of this patent, wedefine grid loss as any condition that interrupts power to the pitchcontrol system of a wind turbine/generator. This can be caused in anumber of ways, including but not limited to, a fault in the main grid;a problem with the pitch control AC power supply (short or other fault);a defective slip ring; a broken conductor, or the like. To understandthe present invention, it is important to note that the pitch controlsystem is traditionally housed within the rotating hub of the turbine.The system needs power to operate. As is well-known in the electricalarts, the most common way of transmitting power from a stationary sourceto a rotating load is via slip rings. It should also be appreciated that“grid loss” as defined herein can occur on either side of the sliprings—on either the stationary or rotating side of the circuit. It isimportant and necessary to detect the loss wherever it may occur, andtake corrective action accordingly. With this in mind, we briefly reviewpatented inventions and published patent applications by others who haveaddressed problems with wind turbines.

U.S. Pat. No. 6,921,985 (Janssen et al.) discloses a low voltageride-through solution for wind turbine generators. The patentedinvention includes a turbine controller and blade pitch control systemwhich are connected to a first power source (AC grid) during a firstmode of operation, and to a second source (backup power) during a secondmode of operation, i.e., during grid power loss. The turbine controllersenses a transition between the two power modes and varies the pitch ofone or more blades in response to the transition. The patent alsoteaches that the turbine controller detects a low voltage event throughcoupling to sensors which provide data indicating the status of variouswind turbine generator system components, for example, rotor speed andgenerator output voltage. When low voltage is sensed the controllertransitions between AC power and UPS power. Janssen et al. measure gridvoltage at the transformer, i.e., on the stationary side of the pitchcontrol circuit. Unfortunately, what this means is that if the inventionof Janssen et al. was to lose a slip-ring, the patented inventionwouldn't detect it.

United States Patent Application Publication No. 2005/0122083 (Erdman etal.) discloses a generator with utility ride-through capability. Thispublication teaches measuring voltage from either a single phase or fromall three phases of the low side of the main grid transformer, butteaches that amplitude of the signal is unimportant. The applicationteaches that frequency and phase are much more important. The systemuses a phase-locked loop scheme to produce a current command signal in ascheme which controls frequency and phase of the generated voltage fromthe wind turbine, and maintains the phase-locked loop signal during abrief fault. Erdmann et al. are silent as to the exact voltagemeasurement point, saying only that, “A frequency and phase angle sensor8 is connected to the utility grid at an appropriate point to operateduring a fault on the grid.” (Paragraph 31). It appears that thereference does not teach measuring at the slip rings on the rotatingside of the pitch control circuit. Also, Erdmann et al. is largelysilent as to powering the pitch control system during a ride through,i.e., the publication doesn't teach a pitch control system arranged tooperate during grid loss.

United States Patent Application Publication No. 2006/0267560 (Rajda etal.) discloses a device, system, and method for providing a low-voltagefault ride-through for a wind generator park, i.e., for a plurality ofwind turbine/generators. The system uses a resistor bank to absorb powerand a control system that maintains collector bus voltage above athreshold voltage during the duration of low-voltage condition on thepower grid. The invention in this application monitors voltage levels onthe collector bus, i.e., the bus coupled through a transformer to thewind turbine driven generator, and not on the rotating side (slip ringside) of the pitch control circuit.

United States Patent Application Publication No. 2007/0057516 (Meyer etal.) discloses a pitch control battery backup method and system. Thepublished application describes an invention which uses a passive methodfor controlling a pitch control system via a charged backup batterywhich provides no power to a DC link when full AC power is available,but uses power from the DC link (including a capacitor) when AC power islost or dips below a threshold level. The patent application is silentas to the method used to sense AC power loss, mentioning “sensor” onlygenerically.

What is needed, then, is a method and apparatus for grid lossride-through for a wind turbine pitch control, and especially for amethod and apparatus that senses grid loss on the rotating side of thepitch control circuit, i.e., proximate the slip rings.

DISCLOSURE OF INVENTION

In a wind turbine/generator having a rotatable hub, at least one bladerotatably secured to the hub, a pitch control system for adjusting pitchof each blade, the pitch control system located within the rotatablehub, a stationary nacelle, and a slip ring assembly at a junction of anelectrical circuit between the rotatable hub and the stationary nacelle,the slip ring assembly operatively arranged for transmission ofelectrical signals between equipment located within the rotating hub andequipment located within the stationary nacelle, an apparatus for gridloss ride-through for the pitch control system, comprising means forsensing and monitoring power on the rotating side of the slip ringassembly, and, means for supplying power to the pitch control systemfrom a backup power source when the sensed power drops to apredetermined level.

A general object of the invention is to provide a method and apparatusfor grid loss ride-through for a pitch control system in a windturbine/generator.

A more particular object of the invention is to provide a method andapparatus for grid loss ride-through for a pitch control system in awind turbine/generator which senses and monitors grid loss (power supplyvoltage) on the rotating side of the slip rings that provide anelectrical connection between the stationary side (nacelle) and therotating side (hub) of the turbine.

Even a more particular object of the invention is to provide a methodand apparatus for grid loss ride-through for a pitch control system in awind turbine/generator which senses and monitors grid loss (power supplyvoltage) on the rotating side of the slip rings that provide anelectrical connection between the stationary side (nacelle) and therotating side (hub) of the turbine, and connects the pitch controlsystem to a backup power supply when the monitored AC supply voltagefalls to a predetermined level. In a preferred embodiment, the systemmeasures all three phase voltages and connects the backup power supplywhen the absolute value of the sum of the squared value of all threephase voltages drops below 80% of nominal line level. The pitch systemcontinues to operate normally even when the AC main voltage has droppedbelow 80%, and as low as zero, for at least as long as three seconds.When the AC main voltage returns to 80% or above nominal, the systemswitches back to AC main supply and disconnects the backup power supply.

These and other objects, features and advantages of the presentinvention will become readily apparent to those having ordinary skill inthe art upon reading the following detailed description of the inventionin view of the drawings and claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained in greater detail below with referenceto the drawings.

FIG. 1 is a side perspective view of a typical prior art wind turbine;

FIG. 2 is a fragmentary perspective view of a section of the windturbine shown in FIG. 1;

FIG. 3 is a fragmentary cross-sectional view of the hub, blades andnacelle, taken generally along line 3-3 in FIG. 2;

FIG. 4 is a cross-sectional view of one of the blades of the windturbine of the invention, shown in a first position, taken generallyalong line 4-4 in FIG. 1;

FIG. 5 is a view of the blade shown in FIG. 4 after the pitch of theblade has been adjusted such that the blade is in a second position;

FIG. 6 is a block diagram of the control circuit for the grid lossride-through pitch control system of the invention; and,

FIG. 7 is a schematic diagram of the power condition monitor and backuptransfer control circuit of the invention;

FIG. 8 is an additional schematic diagram for the Power Loss Detectorsection of the SRF power supply; and,

FIG. 9 is a schematic for the logic power supply of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Adverting now to the drawings, FIG. 1 is a side perspective view of windturbine 10. Wind turbine 10 generally comprises hub assembly 12rotatably secured to nacelle 14, such as by bearings or some othermethod known in the art. The nacelle is mounted atop tower 16, which isof sufficient height to allow hub assembly 12 to fully rotate at a safedistance above the ground. In a preferred embodiment, hub assembly 12comprises three blades 20 rotatably secured to hub 18. The number ofblades, of course, may vary in other embodiments. Wind turbines of thisgeneral structure are known in the art.

FIG. 2 shows a fragmentary perspective view of a section, specificallynacelle 14, of the wind turbine shown in FIG. 1. Wind causes the hubassembly to rotate, which in turn rotates low-speed shaft 19. Thelow-speed shaft terminates in gearbox 24, which is a set of gearsconnecting low-speed shaft 19 to high-speed shaft 28. In a preferredembodiment, gearbox 24 takes a rotational speed of about 30-60 rpm fromthe low-speed shaft and converts it into a rotational speed of about900-1,800 rpm for the high-speed shaft. Generator 26, which could be anysuitable rotational generator known in the art, is attached tohigh-speed shaft 28 to generate electricity.

FIG. 2 also illustrates slip ring assembly 61 on low-speed shaft 19.Slip rings are known in the art as electro-mechanical devices fortransferring electrical currents from rotating sources to stationaryones. In a preferred embodiment brush holders 66 and 68 are proximatethe slip rings to hold the slip ring brushes in place. The disks of theslip rings are secured on a rotatable shaft, so that the disks rotatewith the shaft. A spring or other force constantly presses the brushesagainst the disks so that contact is always made between the disks andthe brushes while the disks rotate freely. As a result, an electricalcurrent can be transferred between rotating and stationary components.

FIG. 3 is a fragmentary cross-sectional view of the hub, blades andnacelle, taken generally along line 3-3 in FIG. 2. Hub 18 is secured tolow-speed shaft 19 such that when wind causes the hub to rotate, the hubin turn causes the low-speed shaft to rotate. Wires 59 a-c deliver powerto the electrical components housed in hub 18. In a preferred embodimentthe electrical components housed in hub 18 are essentially pitch controlsystem 30, but could include additional components. Pitch control system30 may include, but is not limited to, power conversion for pitch motorcontrol module 32, AC to DC conversion module 42, power conditionmonitor and backup transfer controller 48, system control processor 50,backup power source 52, and actuators, sensors, and other controls asnecessary.

FIGS. 4 and 5 show cross-sectional views of two alternate positions forblade 20. In a preferred embodiment blade 20 is rotationally variable ata plurality of intervals between these two positions. Furthermore, itshould be appreciated that it may be desirable in some embodiments toenable blade 20 to be completely rotatable about the axis of the blade.FIG. 4 illustrates blade 20 in a position typical for wind turbine powergeneration. Assuming that wind is coming in a substantially leftward orrightward direction, blade cross-section 22 presents a large surface forwind to strike. The curvature of the blade generates lift, andultimately causes the hub assembly to rotate, as is commonly known inthe art.

In FIG. 5, the pitch of blade 20 has been changed so that blade 20 is ina “feather” position. The blade is very aerodynamic in the featherposition, and allows wind to simply pass over and under the blade, sothat there is no net force on either side of the blade. Therefore, windwill not cause the hub assembly to rotate when the blades are in thisposition. The feather position is typically used in emergencysituations, such as extremely high winds or a grid-loss condition. Pitchcontrol system 30 is housed in hub 18 and responsible for rotating eachblade 20 about its respective axis.

It is desired for many wind turbines known in the art to be able tosense a grid-loss condition, as defined supra, and “ride-through” thegrid-loss condition for a specified period of time, commonly about threeseconds. The ride-through primarily involves providing the pitch controlsystem with power for the specified period of time so that blades 20 canbe moved into a feather position to prevent damage to the turbines.

Electronic Block Diagram & Schematics

FIG. 6 illustrates an electronic block diagram of control system 30 ofthe present invention. It should be noted that FIG. 6 illustrates a“stationary side” of system 30 and a “rotating side”. The two sides areseparated by slip ring assembly 61. As is well known in the art, a slipring is an apparatus for making an electrical connection through arotating assembly, and provides a means of transferring electricity froma stationary to a rotating component. Slip rings, also called rotaryelectrical interfaces, rotating electrical connectors, collectors,swivels or electrical rotary joints, are commonly found in generators,alternators, packaging machinery, cable reels, ceiling fans and windturbines. A slip ring consists of a conductive circle or band mounted ona shaft and insulated from it. Electrical connections from the rotatingpart of the system, such as the rotor of a generator, are made to thering. Fixed contacts or brushes run in contact with the ring,transferring electrical power or signals to the exterior, static part ofthe system. Wikipedia, http://en.wikipedia.org/wiki/Slip_rings.

The equipment on the stationary side of the slip ring assembly islocated in nacelle 14, while the equipment on the rotating side ishoused in hub 18. Power for pitch control system 30 is provided throughtransformer 62. The AC supply to this transformer can come from anysource. It can be provided directly from the main grid through othertransformers, or it can be supplied directly from the generator. In oneembodiment, the AC supply is 690 VAC, and the transformer is configuredto reduce the voltage to either 400 or 230 VAC for transmission acrossslip rings 58.

In the present invention, slip ring assembly 61 comprises slip rings 58and 60. Slip rings 58 are used to transfer supply power across theinterface, while slip rings 60 are used to transfer command and controldata.

The essence of the present invention is that pitch control system 30 isoperatively arranged to sense and monitor AC supply power on therotating side of the slip rings, and to continue to operate for anengineered period of time, in the event of a partial or total loss of ACsupply power. As mentioned previously, the pitch control system of thepresent invention is operatively arranged to operate normally when theAC mains voltage level falls below 80%, and as low as zero, and remainsbelow 80% for at least as long as three seconds, at which time the ACmain level returns to a minimum of 80% of nominal line level. Unlikeprior art pitch control and grid loss ride-through systems, the presentsystem measures supply voltage on the rotating side of the slip rings.When supply power dips to a predetermined level, a backup power supplyis switched on, and continues to supply power to the pitch controlsystem until the main AC supply voltage returns to a minimum level.

Pitch control system 30 broadly comprises AC to DC conversion module 42which is operatively arranged to convert the AC supply voltage to DC. Inone embodiment, module 42 converts the AC supply voltage to 325 VDC.This DC voltage can be either half-wave or full-wave rectified, and isnext provided to DC bus capacitors 34 for filtering and smoothing. Powerconversion for pitch motor control module 32 comprises an IGBT inverteror other device for conversion of the DC supply voltage to appropriateAC voltage supplies, as is well known in the art. A second module 54converts the AC supply to a lower DC voltage, e.g., 24 VDC.

Pitch control system 30 further comprises power condition monitor andbackup transfer control module 48, which is a core component of theinvention. The power condition monitor module measures the three phasevoltages at lines 59 a, 59 b and 59 c. As mentioned previously, this ACsupply voltage is monitored on the rotating side of the slip rings. Thismonitoring scheme has an advantage over prior art methods in that it candetect problems caused by the slip rings, conductors and other parts ofthe circuit that stationary-side monitoring systems cannot detect. Whenthe quality of the delivered power deteriorates according topredetermined algorithms, the power condition monitor disconnects AC toDC conversion module 42 via a command signal sent via line 63, and thensends signals to relay 40 and contactor coil 36 to connect backup powersupplies 46 and 44, respectively. These backup power supplies can takenumerous forms, such as batteries or capacitors, etc. These backup powersupplies continue to power the pitch control system until main AC supplypower is restored (at least to 80% of nominal). It should be appreciatedthat, although in a preferred embodiment, the triggering point forbackup connection and disconnection is 80% of nominal line voltage, thisis not a critical number, and other ranges of voltage drops could beemployed via simple programming, and any number of algorithms could beused to trigger the backup power switchover. Also, in a preferredembodiment of the invention, the power condition monitor measuresvoltage at all three phases, but the invention could easily beconfigured to measure only single phase voltage, or even two of thethree phases.

Also shown in FIG. 6 are two backup power sources 52 and 56,respectively. In a preferred embodiment, power source 52 provides 250VDC to the DC Bus Capacitors, and then to power conversion for pitchmotor control module 32 to power the pitch control motors. Power source56 provides 24 VDC to power various electronic components. For example,the 24 VDC powers brake coils, relay logic, isolated digital I/Osignaling. The voltage is further reduced through conventional voltagedividers, etc., to provide 12 VDC for microprocessors, memory, A/Dconversion, etc., and 5 VDC for CMOS circuits, as is well known in theart. Although the drawing shows two separate backup power supplies, itshould be noted that the backup power supply may comprise a plurality ofbatteries connected in series, with appropriate taps for 12 VDC and 250VDC, respectively. The backup power could also be supplied bycapacitors.

The power condition monitor 48 also communicates with system controlprocessor 50. System control processor 50 also communicates via slipring 60 via serial data communication with turbine control system andinterface to SCADA module 64. Module 64 represents the main turbinecontrol system which communicates with the pitch control system,commanding pitch control, etc.

FIG. 7 is a detailed schematic of the circuitry that monitors the ACline voltage. It shows three separate sections of differential amplifierand absolute value circuits connected in series. The differentialamplifiers are set up with op amps U1A, U2A, and U4A. The absolute valuecircuits are set up with op amps U1B, U2B, and U4B. Absolute valuecircuits are not necessary for the circuitry; they are used in thisinstance just to make a unipolar signal for the Analog to Digital (A2D)converter input. A bipolar signal could also be used; it would just needto be biased to the center voltage of the A2D converters range. So, theabsolute value circuits are just an alternate solution to biasing thesignal to midrange of the A2D converter.

The combination of U1A and U1B sense the voltage of phase 1 of the ACline. U2A and U2B sense phase two, while U4A and U4B sense phase three.Again, in a preferred embodiment of the invention, all three phasevoltages are sensed, although the invention could also be configured tosense only one of the phase voltages, or any two of the phase voltages.The invention could be configured to sense a delta or wye supplyconfiguration, grounded or ungrounded.

There is no special component for the sensing circuit. It uses genericop amps and the A2D converter of a microprocessor. The microprocessorhappens to be from Microchip, but there are many different processors orDSPs that would work.

Examining the phase one sense circuitry, the differential amplifiers aredesigned to have very good common mode rejection through theconfiguration of the feedback networks of R4, R5, and R6 along with somenoise filtering.

The input of TB3 terminal block (section B5) and the differentialamplifiers are configured for various types of AC line configurations.This device can easily be connected to 240 VAC line to line supplies, orto 400 VAC line to line supplies by changing how the signals areconnected to terminal block TB3. Certainly, the device could have theintelligence to sense the voltage connected to it and adjust gainsaccordingly without needing different wiring configurations, or justhave enough A2D resolution to allow for sensing a lower voltage inputsource. But, these are just typical choices on how to implement thegeneral idea while balancing cost, size, complexity, and other concerns.

In sections B2 and B3 there is an input for the status of an emergencysignal. This signal will be used to immediately stop a ride-throughevent and cause the supply to revert back to normal operation even if ithas already sensed a power loss and is operating in its power loss mode.It will also prevent a power loss from being detected if it indicates anemergency previous to power loss detection.

FIG. 8 is an additional schematic diagram for the Power Loss Detectorsection of the SRF power supply. It shows processor, PIC18F4455, fromMicrochip. The processor contains the memory for its program and datavariables. Among the other features contained in this processor are amulti-channel Analog to Digital (A2D) converter, digital inputs andoutputs, and Pulse Width Modulation (PWM) circuitry including captureand compare capability.

This device monitors the AC line voltages by taking A2D conversions ofthe three signals representing the AC line voltages, P1SEN, P2SEN, andP3SEN on pins 19, 20, and 21 respectively. To achieve a fast indicationof low AC line conditions, the processor squares the value of each ofthe AC line conversions and adds the three squared values together. Ifthe incoming AC line has a constant peak voltage for all three phases,then this result will be the same number no matter what time the AC lineis sampled. Thus, the circuit performs a quick sensing of the loss ofthe AC line without needing timing information from the AC line. In apreferred embodiment, the samples and square summation calculation forthis are taken every 500 microseconds, although they could be taken atdifferent intervals. To prevent falsely triggering a power loss event,multiple samples of a low line condition must be detected to cause thepower supply to change its mode of operation.

Signals FC1, FC2, and FC3 (section D/C4) provide a means of selectingcharacteristics of how the supply operates, such as how many samples ofthe AC line are needed to trigger a power loss detection event, what thetrigger voltage of the AC line should be, what the maximum length oftime to operate in the SRF state, or other features or test modes.Similarly, the Serial Communication channel could be used in a similarfashion.

The Analog Test Point section (D3) uses the PWM output capability of theprocessor to generate some analog test points of data internal to theprocessor to view with a multimeter or oscilloscope.

The Serial Communication section (D1/2) can be used to setcharacteristics of the supply as described above, to get the status ofthe supply, to send out live process data of the supply, to interrogatestored information such as peak AC voltages, length of SRF events, howlong the supply has operated and so on, or to reset this stored data.

The Supply Relay Drive (B5) amplifies the output signal of the processorto the level required to operate the relay that connects the 24 VDCsource from the backup power source to the input of the supplies thatgenerate the logic and bias supplies for the pitch system.

The Power Contactor Drive section (B3/4) amplifies the output signal ofthe processor to control the drive circuitry of the main DC bus supplycontactor. This section actually requires the processor to provide twoseparate signals of the correct and opposite polarities in order for themain power contactor to close and remain closed. This is done as anadditional hardware barrier to prevent the main power contactor fromclosing when it shouldn't. This section also uses the output of thereset generator integrated circuit to open the main power contactor ifthe reset generator senses a low level on the 5V supply.

The SRF Status section (C2) amplifies the output signal of the processorto the level needed for sensing by the Pitch Control Processor (PCP).The PCP will notify the Turbine Controller that a power loss has beendetected. The PCP will then monitor the time of the power loss via theSRF Status signal and has the ability to stop the power loss ridethrough prior to the power supply shutting itself down should the ACline power remain lost. If the AC returns to an acceptable level withinthe shortest timeout period, the SRF Status signal will indicate thatthe AC line is once again acceptable for normal operation.

The Supply Status section (A3/4) provides a visual indication of thestate of the SRF power supply.

The A/D Reference section (B1) is the precision reference for the A2Dconverter in the microprocessor.

The Debug Test Points section (B2/3) is intended to aid the debugging ofthe product as it is developed and can also provide test signals forproduction testing.

The Test Mode section (A2) is intended as an additional means forplacing the power supply in various test modes of operation.

Integrated circuit U7 is a reset generator that monitors the 5V logicsupply and generates a fixed reset pulse when the 5V power supplyexceeds the threshold level of the reset generator. If the 5V supply isbelow the threshold level of the reset generator, the processor isplaced in reset, and the Power Contactor Drive is turned off, thusopening the power contactor. See #5 above for additional description ofthe Power Contactor Drive circuit.

J1 (in CO is the interface connector between the Power Loss Detectorcircuit board, and the Logic Power Supply board.

FIG. 9 illustrates a schematic for the logic power supply of theinvention. The logic supply assembly provides bias supply power (+5,+/−12V) for use in the pitch system logic and control circuits. OnboardDC converters PS1 and PS2 produce this power. PS1 and PS2 operate froman unregulated 24V input supply. The pitch control system uses +5,+/−12V and the unregulated 24V to power logic circuits, IGBT gatedrivers, relay logic, and servomotor brake coils.

To maintain servo pitch operation during an AC power outage, the +5V,+/−12V, the unregulated 24V, and the high power servo bus must bemaintained. In a preferred embodiment, the batteries of emergency powerunits of blades 2 and 3 provide the power for this. The emergency powerunit of blade 2 provides 24V battery power for use in the logic supplyassembly. The emergency power unit of blade 3 provides the highvoltage/high current power (225 VDC) for the pitch control servo bus.

On terminal block TB1 is 24V sys. This is the unregulated 24V power thatruns the system relays, brake coils, DC converters during normaloperation when AC power is present. This unregulated power comes from arectified 18 VAC of a control transformer in the system. Also onterminal block TB1 is 24V Bat. This is the battery power that comes fromemergency power unit 2. On the terminal block is 24V out. This is thepower that outputs from the logic supply assembly and powers the IGBTgate drivers. During normal operation with AC power present, 24V sys iscreated by the control transformer, powers relay logic, and brake coilsof the servomotors. It enters TB1 pins 3 and 4, powers the DC convertersPS1 and PS2, and goes back out the 24V out at TB1 pins 5 and 6. In thisstate, relays RL1 and RL2 are open as they are shown in the schematic.

When AC power is lost, the power monitor assembly senses this and sendsa signal to the logic supply assembly to cause relays RL1 and RL2 toclose. This connects the 24V Bat supply at TB 1 pins 1 and 2 to the 24Vsys terminals and also to the 24V out terminals keeping them powered.Now the 24V Bat source of the emergency power unit 2 is powering PS1,PS2, and the 24V out terminal. At the same time, a signal is sent toenergize the power driver circuit of Q2 in the bottom center of thedrawing. Q2 in turn energizes a high power contactor located on thepanel of the control cabinet which connects the emergency power unit 3to the high voltage servo bus, keeping it powered. The result is thatservo operation continues uninterrupted after AC power is lost, beingpowered from the batteries of emergency power units 2 and 3.

When AC power returns, the power loss detector sends a signal to thelogic supply assembly which opens RL1 and RL2. At the same time a signalis sent to turn off the Q2 circuit and de-energize the high powercontactor which connects the emergency power unit 3 to the servo bus.Now the system is being powered from the AC input.

Thus, it is seen that the objects of the invention are efficientlyobtained, although modifications and changes to the invention and to itscircuits can obviously be made by those having ordinary skill in theart, and these changes and modifications are intended to be within thescope of the appended claims.

What is claimed is:
 1. In a wind turbine/generator having a rotatablehub, at least one blade rotatably secured to said hub, a pitch controlsystem for adjusting pitch of each said blade, said pitch control systemlocated within said rotatable hub, a stationary nacelle, and a slip ringassembly at a junction of an electrical circuit between said rotatablehub and said stationary nacelle, said slip ring assembly operativelyarranged for transmission of electrical signals between equipmentlocated within said rotating hub and equipment located within saidstationary nacelle, an apparatus for grid loss ride-through for saidpitch control system, comprising: means for sensing and monitoring poweron the rotating side of said slip ring assembly; and, means forsupplying power to said pitch control system from a backup power sourcewhen said sensed power drops to a predetermined level.
 2. The apparatusrecited in claim 1 wherein said means for sensing and monitoring powercomprises operational amplifiers configured to measure analog voltages,and a microprocessor operatively arranged to convert said analogvoltages to digital signals for further processing.
 3. The apparatusrecited in claim 2 wherein said means for sensing and monitoring powersenses three different phase voltages simultaneously.
 4. The apparatusrecited in claim 2 wherein said microprocessor is operatively arrangedto sum the squares of the sensed voltages and to send a signal toconnect a backup power supply when the absolute value of the sum of thesensed voltages falls below 80% of a nominal level.
 5. The apparatusrecited in claim 1 further comprising means for disconnecting a mainpower source for said pitch control system when said backup power sourceis connected.
 6. The apparatus recited in claim 1 wherein three phasesupply voltages are applied and measured on the rotating side of saidslip ring assembly, and said predetermined level is approximately 80% ofthe absolute value of the sum of the squared value of all three phasevoltages.
 7. In a wind turbine/generator having a rotatable hub, atleast one blade rotatably secured to said hub, a pitch control systemfor adjusting pitch of each said blade, said pitch control systemlocated within said rotatable hub, a stationary nacelle, and a slip ringassembly at a junction of an electrical circuit between said rotatablehub and said stationary nacelle, said slip ring assembly operativelyarranged for transmission of electrical signals between equipmentlocated within said rotating hub and equipment located within saidstationary nacelle, a method for grid loss ride-through for said pitchcontrol system, comprising the steps of: sensing and monitoring power onthe rotating side of said slip ring assembly; and, supplying power tosaid pitch control system from a backup power source when said sensedpower drops to a predetermined level.
 8. The method recited in claim 7,further comprising the step of disconnecting said backup power sourcewhen said sensed power returns to said predetermined level.
 9. Themethod recited in claim 7 wherein said sensing and monitoring power isdone with operational amplifiers configured to measure analog voltages,and a microprocessor operatively arranged to convert said analogvoltages to digital signals for further processing.
 10. The methodrecited in claim 9 wherein said sensing and monitoring power sensesthree different phase voltages simultaneously.
 11. The method recited inclaim 9 wherein said microprocessor operates to sum the squares of thesensed voltages and to send a signal to connect a backup power supplywhen the absolute value of the sum of the sensed voltages falls below80% of a nominal level.
 12. The method recited in claim 7 furthercomprising the step of disconnecting a main power source for said pitchcontrol system when said backup power source is connected.
 13. Themethod recited in claim 7 wherein three phase supply voltages areapplied and measured on the rotating side of said slip ring assembly,and said predetermined level is approximately 80% of the absolute valueof the sum of the squared value of all three phase voltages.